Remove SQLite dependency after migration period

[mrsimpson]

Background

We've successfully migrated from SQLite-based persistence to file-based persistence.

The SQLite dependency (@sqlite.org/sqlite-wasm) is currently kept for backward compatibility to support automatic migration of existing databases.

Tasks

After a reasonable migration period (e.g., 2-3 releases), we should:

• Remove @sqlite.org/sqlite-wasm from packages/core/package.json
• Remove or mark as fully deprecated packages/core/src/database.ts
• Remove packages/core/src/migration.ts (migration utilities)
• Update FileStorage constructor to not need legacy SQLite path format
• Remove migration-related tests
• Update documentation to remove migration references
• Add note in CHANGELOG about removing SQLite support

Migration Period

Suggested timeline:

• Now: v4.2.0 - File-based persistence with auto-migration
• After 2-3 releases: v4.5.0 or v5.0.0 - Remove SQLite completely

Notes

Users who haven't migrated by then can:

1. Downgrade temporarily to migrate
2. Manually recreate their conversation state (it's just metadata, not critical data)
3. The plan files are preserved regardless (most important data)